Chromium-containing materials are widely used in coating compositions because of their advantageous optical-absorption and corrosion-inhibiting properties. In many coating compositions, such as interference coating compositions, layers of chromium-containing materials are used as absorbing layers in multilayer pigment flakes.
For example, as disclosed in U.S. Pat. No. 3,858,977 to Baird, et al., issued on Jan. 7, 1975, in U.S. Pat. No. 5,059,245 to Phillips, et al., issued on Oct. 22, 1991, in U.S. Pat. No. 5,571,624 to Phillips, et al., issued on Nov. 5, 1996, in U.S. Pat. No. 6,132,504 to Kuntz, et al., issued on Oct. 17, 2000, and in U.S. Pat. No. 6,156,115 to Pfaff, et al., issued on Dec. 5, 2000, which are incorporated herein by reference, layers of chromium metal may be used as absorbing layers. As disclosed in U.S. Pat. No. 4,978,394 to Ostertag, et al., issued on Dec. 18, 1990, and in U.S. Pat. No. 5,364,467 to Schmid, et al., issued on Nov. 15, 1994, which are incorporated herein by reference, layers of chromium(III) oxide (Cr2O3) may be used as absorbing layers. As disclosed in U.S. Pat. No. 5,424,119 to Phillips, et al., issued on Jun. 13, 1995, in U.S. Pat. No. 6,235,105 to Hubbard, et al., issued on May 22, 2001, in U.S. Pat. No. 6,524,381 to Phillips, et al., issued on Feb. 25, 2003, in U.S. Pat. No. 6,648,957 to Andes, et al., issued on Nov. 18, 2003, in U.S. Pat. No. 6,759,097 to Phillips, et al., issued on Jul. 6, 2004, in U.S. Pat. No. 6,818,299 to Phillips, et al., issued on Nov. 16, 2004, and in U.S. Pat. No. 7,169,472 to Raksha, et al., issued on Jan. 30, 2007, which are incorporated herein by reference, layers of commonly available chromium-containing alloys, such as Hastelloys, Inconels, stainless steels, and nickel-chromium alloys, may be used as absorbing layers.
Unfortunately, many of the chromium-containing materials in the absorbing layers of prior-art coating compositions are harmful to human health. Chromium metal and chromium(III) oxide, for example, each cause irritation to the skin, eyes, respiratory tract, and gastrointestinal tract. Moreover, these materials may be oxidized to form chromium(VI) species, which are, generally, toxic and carcinogenic. Furthermore, the chromium-containing alloys used in the absorbing layers of prior-art coating compositions, typically, also contain nickel, which is toxic and carcinogenic. Therefore, many prior-art coating compositions based on chromium-containing materials pose potential health and environmental hazards.
Despite their advantageous corrosion-inhibiting properties, in particular, chromium-containing magnetic alloys are not, generally, used as magnetic layers in multilayer magnetic pigment flakes. Rather, as disclosed in U.S. Pat. No. 6,808,806 to Phillips, et al., issued on Oct. 26, 2004, in U.S. Pat. No. 6,818,299, and in U.S. Pat. No. 7,169,472, additional dielectric or insulator layers are, conventionally, used to improve the corrosion resistance of magnetic layers.